1. Field of the Invention
The present invention relates to a manufacturing apparatus for a cathode ray tube for use in a television set, a computer monitor, or the like. In particular, the invention relates to a manufacturing apparatus for a cathode ray tube where the stem of the electron gun and the glass bulb are welded together during the sealing process.
2. Related Art
To produce optimal images with favorable convergence on the phosphor screen of the glass bulb of a cathode ray tube, the axis of the electron gun has to be in alignment with the axis of the glass bulb. A conventional cathode ray tube manufacturing apparatus achieves this by holding the neck of the glass bulb with its axis aligned with the axis of the electron gun during the sealing process where the electron gun is sealed into the neck of the glass bulb. In this process, the electron gun is inserted into the neck in alignment with the axis of the neck, and a sealing burner is used to seal the stem of the electron gun in the neck of the glass bulb.
FIG. 1 shows a conventional manufacturing apparatus that is disclosed by Japanese Laid-Open Patent Application S61-264632. This manufacturing apparatus has a base plate 3 with a bulb receptacle 2 for supporting a glass bulb 1 having a neck 1a so that the axis of the glass bulb 1 is roughly aligned. A bulb securing mechanism 4 is provided on the upper surface of this base plate 3 and a neck chuck mechanism 6 is provided on the lower surface of the base plate 3. The bulb securing mechanism 4 engages the widest-part side surface 1b of the glass bulb 1 at the opposite end to the neck 1a and correctly positions the glass bulb 1 with respect to rotation about the axis of the glass bulb 1. The neck chuck mechanism 6 clamps the neck 1a so that the axis of glass bulb 1 is aligned with the axis of the electron gun 5 that is inserted into the neck 1a.
As shown in FIG. 2, the neck chuck mechanism 6 is composed of a pair of clamping arms 7a, 7b and a spring 10. These clamping arms 7a, 7b cross over one another in the middle and are freely rotatable about the pivot 8. V-shaped engaging grooves 9a, 9b are formed at one end of these clamping arms 7a, 7b, and the spring 10 is stretched between the clamping arms 7a, 7b at the other end to energize the ends featuring the engaging grooves 9a, 9b toward one another.
Due to the V-shape of the engaging grooves 9a, 9b, each of the clamping arms 7a, 7b comes into contact with the neck 1a of the glass bulb in two places, labeled C1, C2 and E1, E2 in FIG. 2. Once the neck 1a of the glass bulb 1 has been clamped by the engaging grooves 9a, 9b in the clamping arms 7a, 7b of the neck chuck mechanism 6 so that the axis of the glass bulb 1 is aligned with the axis of the electron gun 5, the neck 1a of the glass bulb 1 is heated using the sealing burner 11, which results in the neck 1a and the stem 5a of the electron gun 5a being welded together.
This conventional cathode ray tube manufacturing apparatus has the following drawback. When the neck 1a of the glass bulb 1 and the stem 5a of the electron gun 5 are sealed, the heat of the sealing burner 11 causes thermal expansion in the clamping arms 7a, 7b that are clamping the neck 1a. This results in an increase in the distance L between the pivot 8 and the engaging grooves 9a, 9b of the clamping arms 7a, 7b that clamp the neck 1a, and shifts the axis Y of the glass bulb 1 away from the axis of the electron gun 5.
As described above, a conventional neck chuck mechanism 6 is constructed so that the hinged clamping arms 7a, 7b rotate to clamp the neck 1a. When this neck chuck mechanism 6 is used to hold necks with different outside diameters (as when manufacturing different types of glass bulb or when there is variation in the outside diameter of the neck for a same type of glass bulb), such differences will shift the axis Y of the glass bulb 1. In more detail, as the outside diameter D of the neck 1a increases, the angle between the clamping arms 7a, 7b increases in the direction labeled direction B in FIG. 2. This reduces the distance L between the pivot 8 and the position at which the neck 1a is clamped by the engaging grooves 9a, 9b. The opposite is also true, so that as the outside diameter D of the neck 1a decreases, the angle between the clamping arms 7a, 7b decreases in the direction B which increases the distance L between the pivot 8 and the position at which the neck 1a is clamped by the engaging grooves 9a, 9b. This means that changes in the outside diameter of the neck 1a slightly shift the axis of the glass bulb and cause misalignment between the respective axes of the electron gun 5 and the glass bulb 1.